1. Field of the Invention
This invention relates to fluids having high thermal conductivity or low thermal resistivity and their use underground. More particularly, this invention relates to products and methods for dissipating heat underground, particularly heat associated with buried high voltage power lines and other buried electrical transmission and distribution equipment such as cables.
2. Description of Relevant Art
Increasingly, electrical equipment such as high voltage transmission and distribution power lines are being installed (or buried) underground, for safety, ecological, aesthetic, and/or operational reasons. For example, the advantages of buried power lines in tropical regions, where above ground lines are vulnerable to high winds and rains due to tropical storms and hurricanes, are readily apparent. However, the capabilities of such installations are limited by the ability of the installations to dissipate heat generated by the flow of electrical power through the equipment. If the thermal resistivity of the environment surrounding the buried equipment is unsatisfactorily high, the heat generated during functioning of the equipment can cause an increase in the temperature of the equipment beyond tolerable limits resulting over time in the premature failure or destruction of the equipment.
Currently, the industry typically addresses dissipation of heat around buried power lines in one of two basic ways, both of which involve placing a thermally conductive material around the outside of power line cable (whether or not the cable is strung through a carrier pipe). One way uses bentonite grout to which sand may be added to increase thermal conductivity. The other way uses a cement or similar cementitious material containing sand to provide thermal enhancement. The thermally conductive material is typically installed by either digging a trench and backfilling around the cable with the thermally conductive material or by drilling a bore (hole) and then pulling the cable through the bore containing the thermal enhancement material.
Without sand, bentonite grout does not have high thermal conductivity properties. Typical thermal conductivity values for bentonite grouts range from about 0.4 to about 0.6 BTU/hr ft° F. The addition of sand of an appropriate size can increase such thermal conductivity to a range of about 1.0 to about 1.2 BTU/hr ft° F. However, the sand can cause placement problems and high pump pressures as the thermally conductive grout is placed. High pump pressures can lead to a “frac out” situation where the material induces fractures in the soil through which the material can break through to the surface. When the installation is being placed under a roadway, for example, such a “frac out” can cause a highly undesirable “hump” in the road surface. Use of cement grout can magnify such problems. Use of sand can also lead to excessive friction and problems associated therewith. For example, in the case of a pipe bundle containing cables, such friction from sand can result in pulling forces that can exceed the strength of the cable or pipe bundle causing the bundle to separate during installation. Backfilling soil with sand added after the pipe installation might be used to avoid such installation friction but backfilling may not always be possible or effective for the full length of the installation. Further, additional wear caused by the sand to pumps and pump parts remains a concern.